The present invention relates to a heat transfer surface capable of transferring heat by phase-changing liquids which are brought into contact with outer surfaces of its planar plate or heat transfer tube, and more particularly, to a heat transfer surface for use in an evaporator or radiator.
There have been heretofore proposed a number of techniques as to a heat transfer surface for enhancing boiling or evaporating heat transfer. For example, a heat transfer surface covered with a porous layer described in U.S. Pat. No. 3,384,154. The surface having such a porous layer is known to exhibit higher heat transfer performance than that of a conventional smooth surface. However, in such a porous layer, since voids or cavities formed therein are small, impurities contained in boiling liquid contained therein will enter into the voids or cavities to clog them so that the heat transfer performance of the surface will be degraded. Also, since the voids or cavities are thin or narrow, a great amount of boiling liquid will enter into the voids or cavities due to the capillary force thereby cooling the heat transfer surface. As a result, since the generation and growth of vapor bubbles would be suppressed, the heat transfer performance would be degraded in a low heat flux region.
Also, since the voids or cavities formed in the porous layer are non-uniform in size or dimension, the heat transfer performance is locally changed.
On the other hand, as disclosed in U.S. Pat. No. 4,060,125, there has been known a heat transfer wall or surface having a number of tunnels and limited openings.
With such a heat transfer surface, to obtain a high heat transfer performance, it is necessary that a thin liquid film be formed on wall surfaces of the tunnels. In other words, under such a condition that the tunnels are filled with invading liquid or vapor, it is impossible to obtain a higher heat transfer performance. Such a condition of the liquid and vapor in the tunnels is determined by the vapor pressure of vapor bubbles in the tunnels and the fluid resistance of the liquid and vapor at the restricted openings. Namely, the vapor generating rate is decreased at a region where a heat flux is relatively small, so that the vapor pressure in the tunnels is also decreased. Moreover, the number of the restricted openings from which the bubbles will be removed (which will be hereinafter referred to as an "active opening") is decreased whereas the number of the restricted openings into which the liquid will enter (which will be hereinafter referred to as an "inactive opening") is increased. Accordingly, the liquid may readily enter into the tunnel and the interiors of the tunnels are liable to be filled with the liquid. On the other hand, a region where a heat flux is relatively large is kept under a condition essentially opposite to that described above, and the tunnels are liable to be filled with vapor. Accordingly, it is impossible to keep a higher heat transfer coefficient in a wide heat flux range even with the above-described heat transfer surface. In particular, there is a serious problem in performance degradation at a lower heat flux range which has been widely utilized for the industrial purposes.
Also, since the selection of the active and inactive openings depends upon non-uniformity of machining, it is inevitable that the heat transfer performance is greatly changed among individual products.